1. Technical Field of the Invention
The present invention relates generally to a mold for molding resinous domed articles which may be employed as a cover for monitor cameras, and more particularly to a production method of the same.
2. Background Art
Typical monitor cameras employed in stores, banks, etc. are designed to monitor a preselected range across a fixed direction. Cameras of this type have a dead angle so great as not to cover a wide range. In order to avoid this problem, there have been developed monitor devices which turns or pans a single camera horizontally and tilts it vertically for monitoring a wider range. Such prior art monitor cameras are usually suspended from the ceiling and present an appearance persons perceive to be a camera easily, thus giving an unpleasant feeling to customers in the store. Recently, small-sized monitor cameras are, thus, proposed whose major part of the camera and tilt and pan mechanisms are covered with a resinous dome.
FIG. 4 shows such a dome-shaped monitor camera which includes a camera body 1, a mount ring 2, a transparent domed cover 3, an inner cover 4, a camera lens 5, a power supply cable 6, and an output coaxial cable 7. The inner cover 4 is disposed within the domed cover 3 and made from an opaque or a translucent material. The inner cover 4 has formed therein an opening 4a through which the camera lens 5 faces the outside. The power supply cable 6 supplied the power to the camera body 1, a tilt mechanism, and a pan mechanism. The output cable 7 outputs image signals and receives control signals. The domed cover 3 is made from acrylate resin in injection molding and required to have smooth surfaces and a constant thickness for ensuring a given degree of quality of captured images. The domed cover 3 is also required to have substantially a hemispherical surface for vertical and horizontal movements of the camera body 5 over 90.degree. and 360.degree. ranges, respectively.
FIG. 5 shows a conventional mold for forming the domed cover 3 shown in FIG. 4. The mold consists of an upper block 8 (i.e., a female molding block) and a lower block 9 (i.e., a male molding block). The upper block 8 has formed therein a cavity 10 which consists of a hemispherical wall 10a and an annular wall 10b extending straight from an end of the hemispherical wall 10a.
The formation of the cavity 10 of the upper block 8 is achieved by rotating the upper block 8 about a vertical axis V passing through the center 0 of a semicircle having the same radius of curvature as that of the hemispherical wall 10 and rotating and moving a grindstone, which has a surface curved outward with the same radius of curvature as that of the hemispherical wall 10a, from the top to an end of the semicircle. The annular wall 10b of the upper block 8 is grounded by moving the grindstone parallel to the vertical axis V. A hemispherical portion 9a of the lower block 9 may be grounded in substantially the same manner as that of forming the cavity 10, but formation of an annular wall 9b extending straight from an end of the hemispherical portion 9a requires interruption of the grinding and replacement of the grindstone used in forming the hemispherical portion 9a with another one having a flat grinding surface. This is because a comer between the annular wall 9b and a flat upper wall 9c needs to be formed at right angles. The replacement of the grindstone may result in an error in forming a curved outer surface near an end of the hemispherical portion 9a , thereby causing irregularities to be formed on an interface between the hemispherical portion 9a and the annular wall 9b , which will lead to formation of similar irregularities on an inner surface of the domed cover 3 near its end, thereby contributing to distortion of part of images captured by the monitor camera.